I. Field of the Invention
The invention relates to a method of and an apparatus for effecting a smooth transition between adjacent symbol bursts transmitted in different modulation formats. The invention is useful for providing a smooth transition between adjacent GMSK and 8PSK symbol bursts. The invention may be used in an EGPRS system to minimize spurious transmission between adjacent symbol bursts.
II. Description of Related Art
The GSM standard (Global System for Mobile Communications) is a mobile cellular standard that has been used extensively for many years in Europe, Asia and Africa, and, to a lesser extent, in North America. GSM is a TDMA (time division multiple access) system with 8 timeslots per frame and is used for both voice and data operations. In standard GSM the modulation is GMSK (Gaussian Minimum Shift Keying) which has 1 bit/symbol.
As technology has moved on, GSM has evolved in order to keep up. GPRS (General Packet Radio Service) is an extension of GSM in which up to 5 time slots per frame can be used. The time-slots are divided between receive (Rx) and transmit (Tx), for example 4 Rx slots and 1 Tx slot. The EDGE (Enhanced Data rates for GSM Evolution) has also been developed. EDGE uses a higher modulation scheme than the 1 bit/symbol GMSK of GSM. In EDGE, 3 bits/symbol are transmitted. EDGE combined with GPRS is known as EGPRS and is defined by ETSI GSM 05.04.
EGPRS uses two modulation schemes, GMSK and 8PSK (8-ary phase shift keying). The two modulation schemes are different in many ways. The most obvious one is that GMSK has constant amplitude, while 8PSK has variable amplitude. GMSK only modulates the phase and keeps the amplitude constant, while 8PSK modulates both phase and amplitude. One consequence of these differences is that 8PSK has a data transmission rate that is triple that of the GMSK transmitting data rate.
The introduction of EDGE (and GPRS) has enabled a voice call to be made simultaneously with a data call. In practice this requires the mobile station (MS), which is typically a mobile phone, to be able to transmit in GMSK and 8PSK in adjacent time slots. Thus, both modulation schemes will be in the same spectrum and can appear in two adjacent bursts on either the downlink (from base station to mobile station) or the uplink (from mobile station to base station).
In the downlink, the transmitting base station (BS) normally does not ramp down or switch off at end of each time slot or burst, because the BS will need to transmit again in the next time slot. It is desirable for the MS to switch off between transmissions in order to conserve power. However, increasingly there will be two or more adjacent time slots in which the MS will be required to transmit information. Under these circumstances it is more complex to power down at the end of the first burst and power up at the beginning of the next one.
In EGPRS consecutive bursts are separated from each other by a guard period. If the power remains on during the guard period, as it will under the circumstances outlined above, the transmitter has to be controlled carefully in order to minimize interference to others. The EGPRS specification defines a spectrum mask for the transition, which mask is intended to keep spurious emission as low as possible. Although how this is to be achieved is not specified in the standard, the usual approach is to design both the baseband circuitry and the RF circuitry together to satisfy the conditions of the mask. The mask requirements can be readily met if both of the bursts are of the same modulation. Here, the transition can be made smoothly and has been done for some time in GSM base stations even for transitions between different power levels.
However, if the two consecutive bursts are of different modulations, i.e. GMSK followed by 8PSK, or the vice versa, the transition between the two modulations can generate spurious signals at the output stage of the baseband stage of the transmitter, which will be passed on to the RF stage. The resulting RF signal will violate the spectrum mask defined in the standard. While the standard defines the mask to be satisfied, it is silent about how the mask should be satisfied.
As new services are rolled out on the wireless networks there will be increasing demands from users to be able to combine both voice and data calls; the user may decide to make a voice call while surfing the web with the phone in, say, dynamic synchronous transfer mode (DTM). There is therefore a need to address the issue of transitioning between GMSK and 8PSK modulations in adjacent transmission bursts without generating spurious signals outside the bounds of the defined spectrum mask.